The X-ray CT apparatus calculates an X-ray attenuation coefficient of each point from X-ray projection data which is obtained by multidirectional imaging of a subject, and acquires a computed tomographic image (hereinafter, referred to as “CT image”) of the subject. The CT image acquired by this apparatus allows an accurate and immediate diagnosis of a medical condition of a patient at a medical site, and it is clinically useful. However, in order to acquire an image with a high image quality necessary for a doctor to make a diagnosis, a certain amount of radiation exposure is inevitable. In recent years, an influence on the human body exerted by the radiation exposure is perceived as a problem, and there are growing needs for lower exposure. However, as a dose of radiation is lowered to achieve the lower exposure, resulting in decrease of the ratio of signal strength to noise (hereinafter, referred to as “S/N ratio”), and a linear artifact (hereinafter, referred to as “streak artifact”) may occur which causes erroneous diagnosis.
On the other hand, in some cases, the streak artifact may occur due to the subject to be imaged. For example, in such a case that normal-dose imaging is performed on the subject's thorax or abdomen under the condition that both arms come into contact with his or her sides, much X-rays is attenuated in the direction connecting the both arms, resulting in a decrease of signal amount. Therefore, the S/N ratio is decreased and occurrence of the streak artifact is inevitable. This causes a problem that it is not possible to obtain an image with a favorable image quality.
In order to solve the problem above, for example, the patent document 1 and the patent document 2 suggest an adaptive filter. This filter performs smoothing processing on projection data acquired at the time of imaging, and reduces noise that is a primary cause of the artifacts. The patent document 1 discloses a technique that X-ray detecting data is homogeneously smoothed for each channel to reduce the artifacts. The patent document 2 discloses a technique that uses a noise distribution σ approximated from projection data and smoothes the projection data according to a weight of the filter, which is calculated for each channel.